The present disclosure relates to a mixing device for admixing gas or vapor, and liquid in a vessel where a vapor phase and a liquid phase are flowing concurrently. The purpose of the device is to equilibrate the temperature and chemical composition of the outlet mixture exiting the device. The disclosure is suited for, but not limited to, the application of admixing hot hydrogen-rich treatgas and hot hydrocarbon liquid with a cold quench stream between two adjacent beds of catalyst in a hydroprocessing reactor, such as a hydrotreating or hydrocracking reactor.
A large number of mixing devices for two-phase concurrent vessels have been described in literature and patents. The majority of these devices belong to one of the six types given below:
Type 1: Vortex Mixers with Inlet Chutes or Channels Provided in a Collection Tray
An example of such a design is given in U.S. Pat. No. 3,541,000. The mixer comprises a horizontal collection tray plate 6. The collection tray plate is provided with a plurality of sloped chutes 32/34. The entire process stream of vapor and liquid from the catalyst bed above passes through these inlet chutes at high velocity. Below the collection tray is a swirl box 8. The exit jets from the chutes have tangential components and result in a swirling fluid motion inside the swirl box. The fluids then pass over an internal weir 12 and downward through a center opening 10. At the outlet of the opening 10, the cold quench fluid is added through perforated distributor pipes in a spider arrangement 30. A distribution tray 14 is located below the mixer for rough distribution of the liquid. The tray 14 also serves as an impingement plate for the high velocity fluids exiting the opening 10. A distribution tray 4 is located below the rough distribution tray for final distribution of the liquid.
U.S. Pat. No. 4,836,989 describes a mixer similar to the mixer in U.S. Pat. No. 3,541,000. However, for improved mixing of the quench fluid with vapor and liquid from the catalyst bed above, the quench fluid is added through perforated pipe distributors 13 upstream the collection tray 12 instead of downstream.
Examples of patents that relate to vortex types of mixers are: U.S. Pat. Nos. 5,837,208; 5,989,502; 7,045,103; 7,112,312; and U.S. Patent Application Publication 2012/0241006.
Type 2: Swirl Box Mixers with Radial Inlet Flow
An example of such a design is given in U.S. Pat. No. 3,353,924. The mixer comprises a collection plate 6. The cold quench medium is added through a perforated pipe ring 11 above the collection plate. The vapor and liquid from the catalyst bed 3 above the mixer and the quench fluid enter the swirl box 7 through a plurality of inlet ports 8. Unlike the vortex mixer designs mentioned above, the flow through the inlet ports to the swirl box in this type of mixer is mainly in the horizontal/radial direction. The inlet ports are provided with vanes 9 which introduce a swirling motion to the fluids inside the swirl box 7. The fluid exits the swirl box through a center opening 13a. A perforated impingement plate 14 with vertical baffles 16 is provided below the center opening.
Other examples of swirl box mixers with radial inlet flow are given here:
U.S. Pat. No. 3,787,189 describes a swirl box mixer similar to the mixer in U.S. Pat. No. 3,353,924. However, the inlet openings and vanes to the swirl box have a different design, and the impingement plate 23 below the center opening 20 is not perforated. Vanes 22 introducing a swirling motion to the fluids exiting the mixer below the collection plate 18 replace the radial arranged vertical baffles at the mixer outlet.
U.S. Pat. No. 5,462,719 describes a swirl box mixer similar to the mixer in U.S. Pat. No. 3,353,924. The vapor and liquid are first passed through radial perforations in cylindrical baffle 24, then through vanes 22, which results in swirling fluid motions inside the swirl box. The fluids exit the swirl box through the central opening 21 and enter a second mixing box located below the collection plate 20. In the second mixing box, the fluids flow radially outwards and exit the mixer through the radial perforations in cylindrical wall 26.
U.S. Pat. No. 5,534,233 describes another swirl box mixer. Liquid is collected on tray 101, and the vapor and liquid enter the swirl box in a radial direction. Vertical guide plates 105 are used to create a swirling flow before the fluids exit the mixer through the center opening 7. An impingement plate 13 below the center opening, breaks down the high velocity of the stream.
Type 3: Bubble Cap Like Mixers
A bubble cap mixer design is disclosed in U.S. Pat. No. 5,152,967. The mixer comprises a collection plate 16 and a cap 18, 19 overlaying a downcomer 17. The cap and downcomer define the first mixing swirl chamber. The sidewalls of the cap 19 are provided with angled openings. The angled openings cause the vapor and liquid entering the first swirl chamber, to move in a swirling motion. The fluids first flow upward, over the upper edge of downcomer 17, and then downward through the downcomer and a central opening in the plate 16. The mixer is also provided with a second swirl chamber located below the first swirl chamber with inward radial flow.
Other examples of bubble cap like mixers are given here:
U.S. Pat. No. 6,183,702 describes another bubble cap like mixer. The mixer consists of a collection plate 1125, which holds a certain liquid level. The collection plate is provided with vertical baffles 1130, which promote a swirling motion of the liquid on the plate 1125. The swirling motion is further intensified by quench fluid jets exiting from pipes 1140. On the collection tray, a bubble cap like mixer, comprising a slotted cylindrical cap 1150 overlaying a cylindrical downcomer 1165, is mounted over a central opening in plate 1125. The annular space between the cap and the downcomer is provided with semi spiral shaped baffles 1155. The vapor enters the annular space through the slots in the cylindrical wall of cap 1150. The vapor “lifts” the liquid up into the annular space and the vapor and liquid flow upwards through the annular space. Baffles 1155 cause a swirling motion in the annular space. The vapor and liquid flow down through the downcomer and through the opening in the collection plate 1125.
U.S. Pat. No. 8,017,095 describes another bubble cap like mixing device. The mixing device consists of a large bubble cap 85, similar to the bubble cap used in U.S. Pat. No. 6,183,702, located on an annular collection tray 30. Upstream, the bubble cap 85 is a swirl chamber consisting of side walls 42 and 48, inlets 50 and 55, top wall of inlets 46 and 47 and top wall 49.
U.S. Pat. Nos. 3,824,080 and 5,403,560 provide other examples of bubble cap like mixers.
Type 4: Mixers with Separate Mixing of Vapor and Liquid U.S. Pat. No. 5,635,145 discloses a mixer with separate mixing of vapor and liquid. The mixer comprises a collection plate 6 with a center opening. Above the center opening, a vapor swirl box 8 for mixing the vapors is located. The vapor swirl box is provided with apertures 14. The collection plate is provided with other openings with guiding channels 7 to direct the liquid towards the centerline of the reactor. A pre-distribution tray/impingement plate 15 is located below the mixer.
During normal operation, the collection plate 6 holds a certain liquid level and the vapor enters the vapor swirl box 8 and exits through the center opening. The liquid bypasses the swirl box through the parallel liquid channels 7.
U.S. Pat. No. 5,772,970 is another example of a mixing device with separate mixing of vapor and liquid. The mixer consists of collection tray 12 provided with a cylindrical swirl baffle 13, a center opening 14, and vapor chimneys 17. A cylindrical weir 15 is provided at the rim of outlet opening 14. During operation, liquid will collect on the collection tray 12 and the liquid level will build up to at least the height of weir 15. A swirling motion between the swirl baffle 13 and the weir 15 is caused by the tangential liquid inlets 13a and 13b. The liquid overflows the weir 15 and exits through center opening 14. The vapor largely bypasses the liquid through vapor chimneys 17. Part of the vapor may flow through center opening 14 together with the overflowing liquid.
U.S. Pat. Nos. 5,935,413, 7,052,654 and 7,078,002 describe other examples of mixers with separate mixing of vapor and liquid.
Type 5: Baffled Box Mixers with Vertical Flow
U.S. Pat. No. 4,233,269 describes such a design. The mixer consists of an inlet feed duct 12, where the vapor and liquid enter the mixer. From the inlet feed duct, the fluids are passed through two circular mixing orifices formed by doughnut plates 32 and 36 and through one annular flow restriction formed by the disc 34.
Type 6: Baffled Box Mixers with Horizontal Flow
U.S. Pat. No. 7,276,215 describes a baffled box mixer with horizontal flow. The mixer comprises a collection tray 13, a bottom plate 14 with a center opening 25, two-phase inlets 16, and vertical flow baffles 18, 19, and 20, forming a series of contractions and expansions, or a series of mixing orifices. The entire process stream is forced to flow through each mixing orifice at high velocity. A dispersed two-phase flow regime is achieved in each mixing orifice in order to maximize the interphase area between the vapor and the liquid, and thus maximize the heat and mass transfer between the phases. Downstream from each mixing orifice, the expansion results in turbulence and additional residence time. The mixer has a symmetric fluid approach to the outlet opening 25 for improved spread of the liquid to the distribution tray 11, located below the mixer.
U.S. Pat. No. 5,690,896 describes a second example of this type of mixer. The mixer is built as an integral part of the catalyst support system. The mixer collects vapor and liquid in the annular collecting trough 24. Quench fluid is added to the annular collection trough through quench pipes 22 and 23. The vapor and liquid flow through the annular collection trough to the mixing box 30, located between the support beams 14 and 15. The entire process stream enters the mixing box at the inlet 36. The mixing box comprises a single flow channel with 360° turn in the flow direction. After the 360° turn in the mixing box the fluid exits through the center opening 37.
U.S. Patent Application Publication US 2011/0123410 describes a third example of this type of mixer. The mixer comprises collection tray 5 with inlet opening 6, an annular mixing channel 9, and a perforated predistribution tray 11 with a chimney 13. The vapor and liquid pass through inlet opening 6 and annular mixing channel 9, and exit to the perforated pre-distribution tray 11.
U.S. Pat. No. 3,705,016 describes a fourth example. This mixer consists of a screen 11/12 located on a collection and catalyst support plate 8. The screen is covered by inert support material 7. Quench fluid is injected in the catalyst bed above the plate 8. The screen 11/12 allows the vapor and liquid to pass through, while retaining the inert material. After passing through the screen, the vapor and liquid flow vertically through the center opening in collection plate 8. A horizontal mixing box, consisting of a horizontal bottom plate 16 and vertical baffles 20, 21, 22, and 23, is located below the collection plate. The fluids exiting the center opening are first divided into two horizontal streams. Then each of the two streams is again divided into two streams, resulting in a total of four streams. At the mixer exit, two of these four streams are recombined and sent to one side of the reactor cross section, while the remaining two streams are recombined and sent to the other side of the reactor cross section. Finally, the vapor and liquid are distributed through a perforated tray 25.
A last example of a baffled box mixer with horizontal flow is described in U.S. Pat. No. 3,977,834. This patent describes a mixer consisting of a plurality of parallel mixing boxes 13. Each of the mixing boxes is located between a pair of catalyst support beams 7. Quench fluid is added through pipes 11 between the beams upstream from the mixing boxes.
Pressure drop is typically the driving force for mixing in conventional mixer designs. However, in hydrotreating and hydrocracking process units, increased pressure drop in the mixer results in significant additional costs. Examples of this are the increased initial cost of the recycle gas compressor, and increased operating cost in terms of additional shaft power required for the recycle gas compressor. For two-phase mixing, the following general criteria for achieving good mixing and an equilibrated outlet mixture for a given pressure drop have been established:
The mixer needs to have flow restrictions or mixing orifices with high flow velocity and dispersion of the liquid into droplets in order to provide a large interphase area for heat and mass transfer between the phases and to generate turbulence.
The entire process stream needs to be brought together/contacted. It is insufficient to have parallel paths through the mixer, since the parallel streams are not contacted, and an equilibrated temperature and composition of the parallel streams can therefore not be achieved.
The mixer needs areas with lower flow velocity downstream from the mixing orifices to create turbulent flow conditions in the transition from high flow velocity to lower flow velocity and to allow for some hold-up time. Hold-up time is needed for heat and mass transfer. Turbulent flow conditions are needed to mix the phases.
A reasonable distribution or spread of liquid across the reactor cross section must be achieved at the exit or outlet of the mixer. Even if a distribution tray is located below the mixer, a certain liquid spread over the cross section of the reactor is needed at the mixer exit or outlet to prevent excessive liquid level gradients on the distribution tray. For instance, a mixer design exiting all liquid to one side of the reactor would not be acceptable.
Furthermore, the overall mixer height is important. The mixer should be as compact as possible to reduce the height requirement of the reactor/vessel. In a hydrotreating or hydrocracking reactor, room taken up by the mixer cannot be utilized for the active catalyst. A given total volume of catalyst is required in order to convert the reactants into the desired products. Therefore the space occupied by the mixer adds to the required reactor size/height. Hydrocracking reactors are designed for operation up to 200 bar and 450° C., with high partial pressures of both hydrogen and hydrogen sulfide. Typically, the reactors are designed with internal diameters up to 5 meters. Due to the severe design conditions, the hydrocracking reactor has a thick shell, which is typically constructed of 2.25 Cr 1.0 Mo steel, with an internal lining of austenitic stainless steel such as 347 SS. The cost of one meter of reactor straight side is therefore high, and there is a large potential saving from more compact mixer designs.
The type 1 mixers with inlet chutes are among the most commonly used mixer designs in commercial hydrotreating and hydrocracking applications today. These mixers typically employ sloped inlet chutes, and the major part of the mixer pressure drop occurs in the inlet chutes. If properly designed, high flow velocity and a dispersed flow regime will exist in the inlet chutes. The dispersed flow results in a large interphase area available for heat and mass transfer between the liquid phase and the vapor phase. The high velocity also results in a high degree of turbulence downstream from the inlet chutes, which again results in good mixing. Further, the high velocity results in high mass transfer and heat transfer coefficients for heat and mass transfer between the liquid and vapor phases.
The inlet chutes represent parallel flow paths, and the entire process stream is not contacted in the inlet chutes. Therefore, the swirl box of the mixer must be sized to allow for a sufficient number of fluid rotations in order to mix the streams from the different inlet chutes with each other.
The fluid entrance angle α between the flow direction of the fluids entering the swirl box from the inlet chutes and the tangential direction is defined in FIG. 2C. The larger α is, the lower the momentum that is available to establish the swirling motion inside the swirl box, and the lower the number of fluid rotations that is achieved in the swirl box. For many vortex mixer designs of the prior art, the angle α is excessive, and this reduces the number of fluid rotations in the swirl box to the detriment of the mixing performance of the device. See, for instance, U.S. Pat. No. 5,837,208, where the use of a vertical section 27 in spillways 26 increases the angle α significantly. This is illustrated in FIG. 2C.
The diameter Di of the circle of the inlet chutes is defined in FIG. 2B. The diameter Do of the outlet opening is also defined in FIG. 2B. The number of fluid rotations in the vortex mixer, and thus the mixing performance, strongly depends upon the ratio of Di/Do. For many vortex mixers of the prior art, Di is too low. This reduces the diameter ratio Di/Do and thus the number of fluid rotations in the swirl box, and thereby diminishing the mixing performance of the vortex mixer.
The mixing box height Hs is defined in FIG. 2A. In order to ensure a sufficient number of rotations in the swirl box, a larger mixing box height, Hs, will have to be used to compensate for a large α and/or a low Di/Do ratio. As a result, the inter-bed mixer will occupy a larger volume of the reactor, and the size of the reactor vessel will have to be increased, resulting in significant additional costs.
The vortex mixers are characterized by having a good spread of the liquid exiting the mixer due to the high angular velocity of the exiting liquid. The vortex mixer has good turn down capability, since even small vapor and liquid flow rates are normally sufficient to establish the swirling motion in the swirl box.
In the Type 2 mixers with radial inlet flow, the swirl box is characterized by a radial/horizontal inlet flow. The inlet to the swirl box cause a major part of the pressure drop. If properly designed, the inlets will disperse the liquid to generate a large interphase area for heat and mass transfer between the phases. Again, the inlets represent parallel flow paths, and the number of fluid rotations in the swirl box will have to be sufficient to mix the streams entering through the different inlets with each other.
In the Type 3 mixers, the vapor and the liquid take different paths through slots in the cap. The vapor follows a path in the upper portion of the slots, while the liquid takes a path in the lower portion of the slots. The two phases are not contacted efficiently in these inlets/slots. Also, the pressure drop in the inlets/slots corresponds to the pressure drop of the two-phase column inside the upflow channel. This pressure drop is insufficient for dispersing the liquid into droplets. The slots/inlets represent parallel flow paths and the streams from these parallel flow paths will have to be mixed with each other in the upflow channel. The only way to achieve this is if significant swirling motions are introduced in the upflow channel. But due to the low velocity in the inlets, and due to insufficient size of the upflow channel, it is normally not possible to achieve significant swirling motions in the upflow channel. The only location where the entire process stream is contacted is thus in the downcomer of the bubble cap, which is insufficient for equilibration of the temperature and composition.
In the Type 4 mixers with separate mixing of vapor and liquid, all or part of the entire mixer pressure drop is used in parallel mixers for mixing the vapor and liquid separately. Single phase mixing is widely used in the industry in spite of the fact that the controlling step in two-phase mixing is heat and mass transfer between the vapor and the liquid phases.
Each single phase mixer in itself also consists of parallel flow paths like parallel inlet chutes or vanes. In the mixer disclosed in U.S. Pat. No. 5,635,145, there is no two-phase mixing orifice. As a consequence, the two-phase mixing performance of this type of mixer is poor.
The Type 5 baffled box mixers with vertical flow, exemplified by U.S. Pat. No. 4,223,269, provide good mixing performance and fulfill all the criteria for a proper mixer given above. However, this type of mixer requires very large mixer heights, and thus undesirably large reactor/vessel volumes.
The Type 6 baffled box mixers with horizontal flow, as disclosed in U.S. Pat. No. 3,705,016 and U.S. Pat. No. 3,977,834, represent mixer designs with more parallel fluid paths. In the mixer of U.S. Pat. No. 3,977,834, the entire process stream is never contracted in one mixing orifice. In addition, the liquid exit pattern from the mixer of U.S. Pat. No. 3,705,016 is uneven. The type 6 mixer disclosed in U.S. Pat. No. 5,690,896, is a reasonable good mixer, but it does not have expanded flow area sections to generate turbulence in the expansion and to provide hold up time for heat and mass transfer. Also, the fluids approach the center orifice from only one side. The resulting liquid spread at the mixer exit is uneven. U.S. Pat. No. 7,276, 215 represents a very good and compact mixer design and fulfills all the criteria for proper mixing performance given above. However, the turn down capability of all the type 6 mixers is lower than that of the above-described vortex mixers.